The present invention relates to image setting apparatus for providing high quality, two-dimensional images on photosensitive material. More particularly, the present invention relates to a device for transporting photosensitive material past an imaging line in such apparatus to provide a variable first dimension of such two dimensional image on the photosensitive material.
Image setting apparatus of the type to which the present invention relates should be capable of providing image resolutions of up to 2400 dots per inch. This resolution may be achieved by scanning the surface of photosensitive material with the focused spot of modulated beam of electro-magnetic radiation (hereinafter "EMR") such as infrared light. The EMR beam, which may be produced by a laser, is repeatedly scanned in a linear direction across the photosensitive material to provide one dimension of the two dimensional image. The other, perpendicular image dimension is produced by repeatedly moving the photosensitive material, between or during each beam scan, in a direction perpendicular to the aforesaid beam scanning direction.
In order to obtain a resolution of 2400 dpi, the scanning beam must be electronically modulated to achieve this resolution with the particular rate of scan; the photosensitive material must also be incremented with this resolution from scan line to scan line.
The present invention relates to a device which is capable of accurately moving the photosensitive material with this, or any other desired resolution.
Devices for moving photosensitive material - particularly material of prescribed width and arbitrary length - normally include a pair of parallel rollers which are spring biased to press against each other to tightly grip the material at their nip. Either one or both of these rollers is driven by an electric stepper motor through a gear reducing drive such as a worm gear. In a typical device, the gear ratio may be 24:1 so that 24 complete revolutions of the stepper motor are required to produce one rotation of the drive roller. If, for example, the circumference of the drive roller is nominally four inches, twenty four turns of the stepper motor will move the photosensitive material by four inches so that one revolution of the stepper motor will move the photosensitive material by one sixth of an inch. With a conventional stepper motor capable of incrementing two hundred steps in one revolution, a single step of the stepper motor will move the photosensitive material by 1/6 inches times 1/200=1/1200th inches. If the coils of the stepper motor are energized to move the motor by a half step only, it is possible to move the photosensitive material by 1/2400th inches, thus achieving the desired resolution of 2400 dpi in the direction perpendicular to the beam scan line.
Such a photosensitive material transporting mechanism has two drawbacks, however:
(1) Since the device is operating at the limits of its resolution, it is not possible to correct for small errors caused by slippage or stretch of the photosensitive material or for errors in metering this material due to inaccuracy in roller size. Normally, such slip errors and metering errors are repeatable and consistent for any given machine so that it is possible to compensate them by providing small corrections in the stepper motor advance.
(2) Although the device for advancing photosensitive material in increments of 1/2400 inches is also capable of providing lower resolutions by advancing the material by multiple increments between or during successive scans (e.g., two increments, at 1/2400 inches per increment, results in a 1/1200 inch resolution; three increments, at 1/2400 inches per increment, results in a 1/800 inch resolution, etc.) the available lesser resolutions are specific and limited. Consequently, image setting apparatus designed to provide different resolutions in inches will not operate to provide resolutions based on pica's or metric dimensions.
It is therefore a primary object of the present invention to provide a device for transporting and advancing the photosensitive material in an image setting apparatus which eliminates the drawbacks stated above.
It is a specific object of the present invention to provide a device for transporting photosensitive material past an imaging line in an image setting apparatus, which device is capable of correcting repeatable, consistent errors, thereby improving the accuracy of the apparatus.
It is another specific object of the present invention to provide a device for transporting photosensitive material past an imaging line in an image seting apparatus, which device permits the accurate selection and adjustment of resolution in accordance with any resolution standard, thereby to make the image setting apparatus compatable with the various standards in the industry.